1. Field of the Invention
The present invention is directed to the field of a temperature compensated crystal oscillator (TCXO) of a surface mount type and a method of manufacturing the same, and relates particularly to a temperature compensated crystal oscillator which eliminates write terminals provided in a package body for writing temperature compensation data thereinto, thereby promoting a further reduction in outer dimensions.
2. Description of the Related Art
A temperature compensated crystal oscillator which integrates a quartz crystal unit, an oscillation circuit which uses the crystal unit, and a temperature compensating mechanism for compensating the crystal unit for the temperature-frequency characteristic has ability to prevent variations in oscillation frequency caused by variations in ambient temperature. Such a temperature compensated crystal oscillator is widely used as a reference source of the frequency, particularly, in devices used under mobile environments, for example, in portable telephones. In the temperature compensated crystal oscillator, an oscillation circuit and a temperature compensating mechanism is typically integrated into a single IC (integrated circuit) chip.
There are a variety of methods for compensating the temperature compensated crystal oscillator for the temperature. Variations in the resonance frequency of a crystal unit in response to the ambient temperature, i.e., the temperature-frequency characteristic subtly differs from one crystal unit to another. Therefore, for ensuring a fixed oscillation frequency over a wide temperature range in the temperature compensated crystal oscillator, the temperature characteristic of its oscillation frequency must be actually measured after the assembly of the temperature compensated crystal oscillator to create temperature compensation data based on the result of the measurement, and the temperature compensation data must be written into the temperature compensating mechanism within the IC chip, thereby allowing the oscillator to accomplish an optimal compensation for the temperature in accordance with its crystal unit. A conventional temperature compensated crystal oscillator has a crystal unit and an IC chip contained in a package, and write terminals arranged on the surface of the package body, so that temperature compensation data is written into the IC chip from the write terminals.
FIGS. 1A and 1B illustrate a conventional temperature compensated crystal oscillator which has write terminals on the surface of a package body, as described above.
The illustrated temperature compensated crystal oscillator comprises package body 1 for use in surface mounting; crystal blank 2 which functions as a crystal unit; and IC chip 3. Package body 1 is made, for example, of laminate ceramics in the shape of low-profile rectangular solid, wherein recesses 1A, 1B are formed in both principal surfaces, respectively. Therefore, package body 1 has an H-shaped cross-section, as illustrated in FIG. 1A. When package body 1 is surface-mounted on a wiring board, it is placed on the wiring board with recess 1B facing the wiring board.
Crystal blank 2 is accommodated in recess 1A, and cover 5 is bonded to package body 1 to close an opening face of recess 1A, thereby hermetically sealing crystal blank 2 in recess 1A. Crystal blank 2 is, for example, a substantially rectangular AT-cut quartz crystal blank, which is formed with excitation electrodes, not shown, on both principal surfaces, respectively. Extension electrodes extend from this pair of excitation electrodes to both ends of one side of crystal blank 2. Then, both the ends of the one side of crystal blank 2, to which the extension electrodes extend, are secured to the bottom face of recess 1A of package body 1 with conductive adhesive 4 or the like, so that crystal blank 2 is held in recess 1A.
As illustrated in FIG. 2, IC chip 3 comprises oscillation circuit 21 connected to crystal blank 2; and temperature compensating mechanism 22; for generating a temperature compensation signal for compensating crystal blank 2 as a crystal unit for the temperature-frequency characteristic in accordance with the ambient temperature, and supplying the temperature compensation signal to oscillation circuit 21. The two components are integrated in IC chip 3. IC chip 3 is manufactured using a silicon semiconductor substrate or the like through a general semiconductor device fabrication process. In IC chip 3, circuits such as oscillation circuit 21, temperature compensating mechanism 22 and the like are disposed on one principal surface of a semiconductor substrate which forms part of IC chip 3. Therefore, one of both principal surfaces of IC chip 3, which is formed with the oscillation circuit, temperature compensating mechanism and the like, is called “circuit formation surface 3A.”
As Illustrated in FIG. 3, circuit formation surface 3A of IC chip 3 is provided with a plurality of IC terminals 6 along its periphery. IC terminals 6 are provided for electrically connecting parts and circuits external to IC chip 3 to the circuits internal to IC chip 3. Such IC terminals 6 include a pair of crystal connection terminals (XTAL), a power terminal (VDD), an output terminal (OUT), a ground terminal (GND), and for example, four write terminals 6a directed to temperature compensating mechanism 22. Circuit terminals are formed corresponding to the IC terminals on the bottom face of recess 1B of package body 1, so that IC chip 3 is electrically and mechanically connected to package body 1 by securing the IC terminals of IC chip 3 to the associated circuit terminals, for example, through ultrasonic thermo-compression bonding using bumps 7.
Among IC terminals 6, the pair of crystal connection terminals (XTAL) are connected to crystal blank 2 through the circuit terminals, conductor paths, not shown, formed on package body 1, and the aforementioned conductive adhesive 4. Remaining IC terminals 6, including write terminals 6a, extend to the outer surface of package body 1 through conductor paths, not shown.
Among several surfaces of package body 1, mounting electrodes 8 for use in surface mounting are formed at four corners, respectively, of a surface which is positioned to face the wiring board upon surface mounting. Then, among IC terminals 6, the power terminal (VDD), output terminal (OUT), and ground terminal (GND) extend to mounting electrodes 8, respectively, by way of through-holes, not shown, formed through four corners of package body 1. Four write terminals 6a of IC chip 3 are electrically connected to external write terminals 6A formed on a side surface of package body 1 through conductor paths. Two of external write terminals 6A are arranged on, for example, each of one pair of side surfaces along the longitudinal direction of package body 1.
For protecting circuit formation surface 3A of IC chip 3, protective resin 9 is injected as so-called under-fill between the bottom face of recess 1B and circuit formation surface 3A in recess 1B of package body 1.
In the temperature compensated crystal oscillator as described above, oscillation circuit 21 is operated, while crystal blank 2 is accommodated in recess 1A to make up a crystal unit, to measure the characteristic of the oscillation frequency with respect to the temperature. Then, temperature compensation data is created for compensating the crystal unit for the frequency-temperature characteristic based on the result of the measurement, and such temperature compensation data is written into a memory circuit of temperature compensating mechanism 22 within IC chip 3 from four external write terminals 6A arranged on the side surfaces of package body 1. After the temperature compensation data has been written, temperature compensating mechanism 22 generates a compensation voltage adapted to the frequency temperature characteristic of crystal blank 2 in accordance with the ambient temperature, and this compensation voltage is supplied to oscillation circuit 21. Oscillation circuit 21 employs, for example, a voltage variable capacitance element as part of a load capacitance to the crystal unit, so that the load capacitance of the crystal unit varies in response to the ambient temperature as the compensation voltage is applied to the voltage variable capacitance element, thus accomplishing the compensation of the oscillation frequency for the temperature
The temperature compensated crystal oscillator described above encounters increasing difficulties in the formation of four external write terminals 6A on the side surfaces of package body 1 as its outer dimensions are reduced more and more.
When the temperature compensation data is written, probes, not shown, used for writing data, are brought into contact with external write terminals 6A, and the data is written from the probes. Therefore, each of external write terminals 6A requires a certain area or more in order to establish an electric contact with the probe. However, with the advancement of a reduction in the outer dimensions of the temperature compensated crystal oscillator, required areas cannot be ensured for external write terminals 6A, and an electric contact can be induced between external write terminal 6A and mounting electrode 8 due to a narrower spacing therebetween. Further, the reduction in the outer dimensions can cause difficulties in routing conductor paths for connecting write terminals 6a of IC chip 3 to external write terminals 6A. For these reasons, it is increasingly difficult to form four external write terminals 6A on the side surfaces of package body 1.
Japanese Patent Laid-open Application No. 2001-36343 (JP, P2001-36343A) discloses an example of a surface-mount temperature compensated crystal oscillator which is provided with external write terminals on side surfaces thereof. Also, Japanese Patent Laid-open Application No. 2002-190710 (JP, P2002-190710A) discloses an example of a surface-mount crystal oscillator which has terminals for measuring the characteristic of a crystal unit on an outer surface of a package.